The invention is in the field of digital image processing and is directed to a method for generating a frame for a graphic object whose contour is described by a sequence of Bezier curves. The generation of a frame for graphic objects is a job that occurs in many fields of application of digital image processing.
An important field of application is electronic reproduction technology wherein print masters for printed pages are produced, these containing all elements to be printed such as texts, graphic objects and images. In the case of electronic production of the print masters, these elements are present in the form of digital data. The data are generated for an image, for example, in that the image is scanned point-by-point and line-by-line in a scanner, each element is resolved into color components, and the color values of these components are digitalized. The data for texts and graphic objects are generally generated directly in a computer by text processing and drawing for drafting programs. Dependent on the output process employed later, for example, output on a color printer or printing in a conventional printing machine, the data for the page elements are generated and stored in the color components red, green and blue (RGB) or in the inks of 4-color printing of cyan, magenta, yellow and black (CMYK).
During the further work sequence, the digitalized texts, graphic objects and images are assembled to form a printed page in a processing station either under visual control on a color monitor or automatically according to stored layout rules. The printed page data are then converted into a data format suitable for the output, for example, into the page description language PostScript and are stored. Separate printed page data, which are referred to as color separation data, are thereby produced for each of the inks in CMYK. The color separation data are output with a film or plate recorder in high resolution on film material or directly on printing plates. There are also digital printing machines that work without printing plates. In this case, the color separation data are directly transferred to the digital printing machine and are printed thereat on the printing material.
Before the recording, the printed page data are checked at a processing station, and are modified in many ways according to the rules of a layout, and are processed according to the manuscript, for example, corrected in color, are positioned on the printed page, etc. A frequent modification is thereby the production of a frame for a graphic object. What is hereby understood by a graphic object is an area having an arbitrary shape whose contour is described in terms of data by a sequence of curves and straight sections, and that is filled with color values in an arbitrary way. In the simplest case, the area is filled with a constant color. However, it can also contain a color progression, i.e., a continuous variation of the colors in a specific chromatic range. A graphic object can also contain scanned image data or a pattern of different color values. It is of no significance in conjunction with the present invention as to what colors are contained in the area of the graphic object and how they are distributed. The only thing of significance is that the contour of the graphic object is described by a sequence of curved and straight sections and is to be framed.
What is achieved by the framing of a graphic object and the coloration of the frame with a contrasting color is, for example, that it is more clearly contrasted compared to neighboring, graphic objects or compared to the background. Another important application for the production of a frame around a graphic object in the area of reproduction and printing technology is what is referred to as trapping. A frame is thereby produced in only some color separations of the graphic object but not in other color separations. What is thus achieved is that, given shifts of the color separations relative to one another in the printing machine (registration errors), no disturbing white edge of unprinted paper can be seen at a part of the contour of the graphic object on the finished printed page. What is assured by the trapping of the correct color separations is that an adequate great overlap of the graphic object with its environment is still present given such shifts, so that all locations of the printed page are printed. Those color separations for which a frame is generated and those for which a frame need not be generated are dependent upon the colors within the graphic object in relationship to the colors in its immediate environment.
According to the method of the present invention for framing a graphic object whose contour is described by cubic Bezier curves by producing an approximately parallel, cubic Bezier frame curve having checkpoints Q0, Q1, Q2, Q3 for each given cubic Bezier curve of the contour having checkpoints P0, P1, P2, P3, the checkpoint Q0 is defined proceeding from the checkpoint P0 perpendicular to a line P0-P1 at a spacing of a frame width. The checkpoint Q3 is defined proceeding from the checkpoint P3 perpendicular to a line P2-P3 at the spacing of the frame width. The checkpoint Q1 is placed onto a first parallel that proceeds through the checkpoint Q0 parallel to the line P0-P1. The checkpoint Q2 is placed onto a second parallel that proceeds through the checkpoint Q3 parallel to the line P2-P3. A midpoint PM for the given Bezier curve is constructed from the checkpoints P0, P1, P2, P3. A midpoint QM for the Bezier frame curve to be produced is determined proceeding from the mid-point PM perpendicular to the given Bezier curve at the spacing of the frame width. A position of the checkpoint Q1 on the first parallel and the checkpoint Q2 on the second parallel is calculated from a condition that the mid-point QM can be constructed in the same way from the checkpoints Q0, Q1, Q2, Q3 as the mid-point PM is constructed from the checkpoints P0, P1, P2, P3.